1. Field of the Invention
The present invention relates to a thin-film coating apparatus, and more particularly, to a thin-film coating apparatus, including a solution dropping nozzle device, for forming a metal oxide film or diffusion source film on the surfaces of materials to be treated.
2. Description of Relevant Art
A variety of known types of thin-film coating apparatus are employed for forming a thin-film consisting of photoresist, metal oxide, diffusion source, and the like on the surfaces of materials to be treated. A variety of known types of nozzle devices are employed in such apparatus for the dropwise application of the coating solution onto the materials to be treated. To apply a diffusion source onto a wafer, for example, in a fabrication process of an IC, LSI or the like, as illustrated in FIG. 4 of the accompanying drawings, a wafer 51 is mounted on a spinner 50, a coating solution containing a diffusion source is applied by dropping same onto a central surface portion of the wafer 51 from a nozzle 52, and the wafer 51 is then spun at a high speed by the spinner 50 so as to provide a uniform coating of the diffusion source on the surface of the wafer 51 by virtue of the centrifugal force.
After dropping the coating solution from the nozzle 52, a small amount of the coating solution still remains at a peripheral edge portion 52a of the nozzle tip due to surface tension. Because the coating solution is generally prepared by dissolving a diffusion source in a solvent such as an organic solvent of relatively high volatility, in just a short time only the solvent evaporates from the coating solution remaining at the peripheral edge portion 52a of the nozzle tip. There thus results a gradual concentration of the coating solution and eventually the deposition of the solute, i.e., the diffusion source.
When the thus concentrated coating solution or the thus formed deposit drops onto the wafer being treated, an uneven film is applied to the wafer, thereby making the film defective. Also, when the deposited solute is directly exposed to the atmosphere, the solute is likely to chemically react thereto, thereby producing insoluble material which necessarily disturbs the treatment. This is highly disadvantageous in consideration of the quality of the finished product.
The aforesaid disadvantage is also encountered when forming a metal oxide film on the wafer, because in this case the coating solution is also generally prepared by dissolving an agent for forming a metal oxide, such as tetraalkoxisilane, in an organic solvent of relatively high volatility. However, the aforesaid disadvantage is rarely encountered when forming a photoresist film on the wafer, because in this case the solvent for preparing the coating solution is of relatively low volatility.
Specifically, the kinds of solvents for preparing the coating solutions including the photoresist, diffusion source and agent for forming metal oxide are as follows, wherein the evaporation rate per a unit time of each solvent is indicated by means of proportion assuming the proportion of evaporation rate of n-butyl acetate (normal butyl acetate=C.sub.4 H.sub.9 OCOCH.sub.3) as 100, and noting that each solvent has been employed in practice by virtue of its reaction and coating characteristics:
______________________________________ Proportion of Evaporation Rate Solute Solvent of the Solvent ______________________________________ Diffusion methyl alcohol 460 source, or ethyl alcohol 190 agent for n-propyl alcohol 110 forming iso-propyl alcohol 170 a metal sec-butyl alcohol 120 oxide ethyl acetate 590 n-propyl acetate 230 iso-propyl acetate 159 n-butyl acetate 100 iso-butyl acetate 140 sec-butyl acetate 200 Photoresist ethylene glycol mono- 20 ethyl ether ethylene glycol mono- 10 butyl ether ethylene glycol mono- 25 ethyl ether acetate ethylene glycol mono- 3 butyl ether acetate ______________________________________ NOTE: The percentage evaporated (at atmospheric pressure) of nbutyl acetate is 100% in 7.9 hours.
In order to avoid the aforesaid disadvantage, the dropping of a concentrated coating solution or deposit has been prevented conventionally by wiping the nozzle tip portion with sponge, cloth, filter paper or the like, which may optionally be impregnated with a solvent. However, such a conventional method necessarily relies upon troublesome manual operations, and thus involves problems from the standpoint of mass productivity. In addition, it is rather difficult to conduct such wiping-off operation where the spacing between the nozzle and the spinner is not sufficient, thereby possibly leading to an accidental dropping of a foreign material onto the surface of the wafer. Furthermore, such a conventional method does not permit the carrying out of the coating step and its preceding and subsequent steps as a series of continuous operations, thereby impeding the full automation of a fabrication process.
The present invention effectively overcomes the foregoing problems and disadvantages attendant the conventional techniques.